There are three issues with airborne cell phones:
1: interference with multiple cells.
2: interference with avionics
3: interference with (distraction of) the pilot

All are significant. #1 was much more significant with analog phones.
One day in 1995 I was giving a glider ride, had caught a good thermal,
and the analog rang. I answered, talked to the nursing-home nurse for
a bit; the thermal was strong; as I climbed through 4K agl, suddenly
ther was a loud electronic squeal and the conversation ended. More
recently I needed to make an urgent call from an airplane with a
digital phone, and got the same squeal.
#2 is randomly significant. A pilot friend told me last month, "I was
on the ramp ready for departure and placed a call on my cell phone.
When I hit 'send' all the radio displays zero'ed out. That made a
believer out of *me*!"
#3 is self evident and has been mentioned by several posters already.